The most prevalent technologies for the object identification are one- or two-dimensional bar codes. Because the barcode needs space to arrange the ordered data, either in a one-dimensional bar-sequence or two-dimensional image, the barcode reader has to scan the bar-sequence or register the image. These procedures make the system bulky and complicated. In addition, the visibility of the printed pattern of a barcode is vulnerable to counterfeiting, making barcodes useless in security applications.
Multiplexed optical coding technology has been reported for bioanalytical application. In this technology, the spectral features, namely wavelength and intensity, of fluorescence generated from multiple fluorescent substances are employed for coding information. Organic dyes and metal complexes are commonly used as fluorescent materials in various applications. In principle, they are applicable to the proposed multiplexed spectral coding technology (e.g., using multiple wavelengths and multiple intensities). However, they generally have inadequate adsorption and emission properties. Different exciting light wavelengths are required to excite a mix of multiple fluorescent molecules and the emission spectra are either broad or asymmetrical, making the information retrieval difficult. Among other problems encountered are also the possible interactions between two different fluorescent molecules and the immiscibility of the multiple fluorescent molecules in a common matrix material. From technical point of view, an ideal set of luminescent substances should have the following properties in order to meet the multiplexed coding:                Strong, single wavelength or mono-dispersed narrow emission for each individual luminescent substance;        Emission spectrum independent of the exciting light in certain range of exciting wavelength;        Single light source for all luminescent substances;        No interaction among different fluorescent molecules, i.e., each luminescent substance responds to the exciting light independently;        No influence of matrix material on emission; and        Miscibility of all luminescent substances in the selected matrix materials        
The idea of using semiconductor nanocrystals for spectral coding of biomolecules has been proposed by Han et al. “Quantum-dot-tagged microbeads for multiplexed optical coding of biomolecules”, Nature Biomtechnology, 2001, 19, 631–635.